1. Field of the Invention
This invention relates to cooling towers and particularly to a molded, composite airfoil-defining, synthetic resin blade for large diameter cooling tower fans.
2. Description of the Prior Art
Industrial size induced draft water cooling towers have one or more relatively large diameter fans which pull in air from the surrounding atmosphere and direct such air through the water to be cooled by evaporative effect, before discharge of the hot air through a velocity recovery stack. Fans for these applications generally are of a diameter within the range of from about 12 feet to as much as 60 feet or more.
Small diameter cooling fans within the range of from 2 feet to 12 feet in diameter have for the most part been made of metal such as aluminum. Large diameter industrial cooling tower fans having diameters of from 12 feet to as much as 60 feet on the other hand have often been manufactured from fiberglass reinforced synthetic resin in order to reduce the overall weight of the blade and hub assembly. In small diameters, cooling fan blades of aluminum are less expensive than plastic blades. However, for industrial size fan blades, design constraints often preclude the use of aluminum or other metals. Plastics, usually reinforced with materials such as fiberglass, are the construction materials of choice. Aluminum blades for example become too heavy where the blades are to be used in fans having a diameter of 20 feet or more.
Tip speeds of cooling tower blade fans are relatively high and can be anywhere from 10,000 feet per minute to about 15,000 feet per minute. As a consequence, a synthetic resin cooling tower blade must be capable of withstanding tip speeds of this magnitude over extended usage periods without degradation of the blade surface or its internal components. This is a particularly difficult problem to overcome because of the extremely humid conditions encountered in cooling tower fan operation. Abrasion of the surface of the blade can also occur because of foreign materials in the air stream created by the fan.
Plastic fan blades made up of synthetic resin material reinforced with glass fibers have for the most part been manufactured of an epoxy resin containing fiberglass reinforcement. However, the cost of the resin and the limitations on the use of thermoset type resins such as epoxies, have made epoxy blades very expensive to manufacture and difficult to sell with a reasonable return on the investment.
Polyester fan blades, on the other hand, are less expensive because of the lower price resin, but it has not been heretofore feasible to fabricate polyester having physical and chemical properties commensurate with those of epoxies.
Difficulties have also been encountered in providing an effective means for securing a plastic blade to the steel hub of the fan gear box. The blade shank adapted to be attached to the fan hub must not only be rigidly affixable to various types of hubs, but most importantly, there must not be any tendency for the shank to move relative to the blade which would result in relatively rapid deterioration of that end of the blade and render the latter inefficient at best, and totally ineffective at worst by virtue of dislodgement of the shank from the blade itself.
Manufacture of large diameter plastic fans presents a formidable challenge though because of the tendency of the blade tips to oscillate in use under certain ambient wind conditions and at a particular rpm value, which can damage the blade if tip excursion becomes great enough, and also interferes with efficient blade function.
A need thus exists for a reasonably priced plastic blade for large diameter industrial water cooling tower fan applications where a good surface finish can be provided, the blade has necessary strength characteristics, the required compound curves can be formed, adequate strength to weight ratios can be obtained, and required longevity can be assured. Heretofore, these requisites have not been obtainable at a competitive price.
Composite aircraft propellers manufactured of synthetic resin reinforced with glass fiber material and formed over foam cores have been available for a number of years but the problems presented in the manufacture of aircraft blades are significantly different from those encountered in the design and fabrication of significantly longer blades used in industrial cooling towers. Examples of composite aircraft blades are illustrated and described in Hartzell Propeller, Inc., U.S. Pat. Nos. 4,302,155 and 4,810,167. Aircraft propellers though sell for a significantly higher cost on a linear basis than can be charged for industrial water cooling tower fans and thus it is not commercially practical to employ the technology that has been developed and is in use for manufacture of water cooling tower fan blades.